The Construction And Electrochemical Properties For High-perfomance Alkaline Secondary Ni-Fe Batteries

Large-scale rechargeable batteries with high power and high energy while offering low cost,long lifetime,high safety and eco-friendliness are of great importance and highly demanded for applications in electrical vehicles and large portable devices.Lithium-ion and lead-acid batteries are currently mainly available in the market for the large-scale electric power sources but the former is not cost-effective and the later suffers greatly from environment and safety concerns.The traditional aqueous batteries?ABs?are cost-effective and safe,and have attracted intensive research activity to develop high power and energy storage devices as alternatives for lithium-ion batteries.Different kinds of ABs have been reported,including MnO₂/Zn,LiMn₂O₄/VO₂,FeF_x/Co?OH?₂,Ni/Fe and so on,which are comparable to LIBs in specific energy.Among them,Ni-Fe ABs have some prominent advantages owe to their high energy density,abundant metal resources in global stockpile and easy recycle.However,current Ni-Fe ABs suffer from relatively poor iron anode with low energy density and poor stability,which needs considerable efforts for improvement of the performance.In view of the multiple valences(Fe³⁺???Fe⁰)and rich redox chemistry of iron element in the negative potential window,iron oxides?FeO_x?,such as FeO,Fe₂O₃ and Fe₃O₄,exhibit low-cost,harmfulless and environmentally friendly nature,and thus are promising anode materials for Ni-Fe ABs.However,FeO_x as the electrode material usually exhibits poor electrical conductivity(¹0^-1414 S cm^-1)and easy passivation by iron hydroxide during the discharge process,which severely hinder the charge storage capability.Considering that three-dimensional?3D?porous graphene hydrogels?GHs?not only can allow rapid multidimensional electron transfer and accelerate ion diffusion in the network,but also can bring more active sites for electrochemical reactions without addition of binder and conductive carbon black,we make efforts to modify FeO_x with graphene hydrogels.Unfortunately,current typical 3D graphene hydrogels were prepared by chemical reduction of graphene oxide?GO?dispersion,in which toxic organic solvents?DMF,ethylene glycol,methyl alcohol,acetone,acetonitrile,etc.?and toxic reagents?NH₃?H₂O,ammonia,hydrazine hydrate N₂H₄,etc.?were used to form hydrogels and transform GO into rGO.It is of long-term significance to investigate a new way to replace this ineconomical and environmentally unfriendly strategy and find a kind of high-performance positive electrode matching material for the preparation of environment-friendly alkaline secondary nickel-iron batteries.This thesis mainly investigates the following three aspects:?1?Synthesis of Fe₂O₃/graphene hydrogels?Fe₂O₃@GHs?.There are numerous deficiencies in the current method for synthetic graphene hydrogel,such as long reaction time,use of toxic reagents,complex instruments and high temperature or pressure.In chapter 3 and 4,we study the effect of phase to electrochemical performance and find out the best Fe₂O₃@GH with defects from hydrothermal method.The as-prepared hybrid hydrogels exhibit a high specific capacity of 304.2 mAh g^-1 at a current density of 1 A g^-1,which is superior to pure Fe₂O₃ powder(97 mAh g^-1)and conventional FeO_x/C(287 vs.159.5 mAh g^-1 at 2 A g^-1).The capacity of bare Fe₂O₃electrode declines to less than 20%of capacity retention,whereas Fe₂O₃@GH electrode retain⁹5%of capacity after 1000 cycles.The outstanding electrochemical activity and satisfying stability of Fe₂O₃@GH,to some extent,can be owed to negligible agglomeration and enhanced interface contact.?2?Preparation of a high-performance secondary battery cathode material.In chapter 5,nickel hydroxides with different morphologies are synthesized.The best electrochemical performance is shown by nickel hydroxide microspheres?Ni?OH?₂MSs?which are directly grown on foam nickel?Ni?OH?₂ MSs@NF?.The Ni?OH?₂MSs@NF shows a specific capacity of 313 mAh g^-1 at the current density of 1 A g^-1 and ⁰.65? of equivalent series resistance?R_s?and¹.0?of charge-transfer resistance(R_ct).Apparently,the high capacity and good rate capability are ascribed to the outstanding conductivity and porous nanostructure,which facilitate the efficient electron transfer and fast ion diffusion.?3?Development of an alkaline secondary nickel-iron battery with high energy density and low environmental concerns.A rechargeable aqueous Ni-Fe battery assembled with Fe₂O₃@GH?-?//Ni?OH?₂@Ni foam?Ni?OH?₂ MSs@NF??+?produces an energy density of 203 Wh kg^-1 and power density of 6.4 kW kg^-1 as well as remarkable cycling stability with retention of 82%of specific capacity after 500 cycles at a large operating potential of 1.6 V.These excellent performances indicate that the alkaline battery has practical application value.In summary,high-performance Fe₂O₃@GH and high-capacity Ni?OH?₂microspheres have been successfully synthesized.Then,the two are perfectly matched and a high power density battery of Fe₂O₃@GH?-?//Ni?OH?₂MSs@NF?+?is prepared.This battery can meet the demand for high energy density and high power density for electric vehicles and smart electronic devices.The research contributes to the development of green power system.